HK navy units are the Hunter Killers which were designed to destroy humans resistance naval bases around the world including Skynet battle ship and anti-submarine warfare machine.
The Kraken is a class of antisubmarine warfare machine created by Skynet and used to patrol Earth's oceans in 2027. It was unable to dive as deep as a Seawolf submarine and normally followed a set patrol route.
On September 13, 2027, while on a mission from John Connor himself, the USS Jimmy Carter was engaged by a Kraken.
Originally the crew believed it to be an ASW Bot; however, the size difference depicted on sonar confirmed that this wasn't the case. The Kraken was tricked into believing that the Jimmy Carter had been destroyed due to quick thinking by its captain, Queeg, a reprogrammed Series 888 Terminator.
The machine's name is a reference to the Kraken, a giant sea monster of legend.
An ASW Bot was an antisubmarine warfare unit designed by Skynet and in use by, at least, the year 2027. They patrolled the Earth's oceans in pairs looking for human controlled submarines and ships. However, the story in TRFC, explained that an ASW Bots looked somewhat like a hydrobot but but it was bigger than hydrobot and it has missile launchers on their heads, not metal fangs like hydrobot.
During a supply mission, the USS Jimmy Carter came under attack by what was believed to be a group of ASW Bots. However, it turned out that they were being attacked by a Kraken.
A Hydrobot is a Non-Humanoid Hunter Killer unit produced by Skynet sometime before 2018. It features a serpent-like structure with a claw at their head. It is used to patrol the waters. The Hydrobot is based on the present day model of Skynet Research's Aquatic WorkBot, which was used to maintain kelp and seaweed, find minerals and oil, and seek out organic matter.
As the name implies, a Hydrobot has difficulty operating on land, but they could jump incredible distances and use their metal teeth to cut through even metal.